What's the next number

What's the next number

The rules are as follows: 1 11 --- denotes that the previous number "1" is a 1; 21 --- denotes that the previous number "11" is composed of two 1; 1211 --- denotes that the previous number "21" is composed of one 2 and one 1; 111221 --- that is, 11 12 21, denotes that the previous number "1211" is composed of one in turn